Pain is a protective mechanism that allows healthy animals to avoid tissue damage and to prevent further damage to injured tissue. Nonetheless there are many conditions where pain persists beyond its usefulness, or where patients would benefit from inhibition of pain. Neuropathic pain is a form of chronic pain caused by an injury to the sensory nerves (Dieleman, J. P., et al., Incidence rates and treatment of neuropathic pain conditions in the general population. Pain, 2008. 137(3): p. 681-8). Neuropathic pain can be divided into two categories, pain caused by generalized metabolic damage to the nerve and pain caused by a discrete nerve injury. The metabolic neuropathies include post herpetic neuropathy, diabetic neuropathy, and drug-induced neuropathy. Discrete nerve injuries indications include post amputation pain, post-surgical nerve injury pain, and nerve entrapment injuries like neuropathic back pain.
Voltage-gated sodium channels (Nav's) play a critical role in pain signaling. Nav's are key biological mediators of electrical signaling as they are the primary mediators of the rapid upstroke of the action potential of many excitable cell types (e.g. neurons, skeletal myocytes, cardiac myocytes). The evidence for the role of these channels in normal physiology, the pathological states arising from mutations in sodium channel genes, preclinical work in animal models, and the clinical pharmacology of known sodium channel modulating agents all point to the central role of Nav's in pain sensation (Rush, A. M. and T. R. Cummins, Painful Research: Identification of a Small-Molecule Inhibitor that Selectively Targets NaV1.8 Sodium Channels. Mol Interv, 2007. 7(4): p. 192-5); England, S., Voltage-gated sodium channels: the search for subtype-selective analgesics. Expert Opin Investig Drugs 17 (12), p. 1849-64 (2008); Krafte, D. S. and Bannon, A. W., Sodium channels and nociception: recent concepts and therapeutic opportunities. Curr Opin Pharmacol 8 (1), p. 50-56 (2008)). Nav's are the primary mediators of the rapid upstroke of the action potential of many excitable cell types (e.g. neurons, skeletal myocytes, cardiac myocytes), and thus are critical for the initiation of signaling in those cells (Hille, Bertil, Ion Channels of Excitable Membranes, Third ed. (Sinauer Associates, Inc., Sunderland, Mass., 2001)). Because of the role Nav's play in the initiation and propagation of neuronal signals, antagonists that reduce Nav currents can prevent or reduce neural signaling and Nav channels have long been considered likely targets to reduce pain in conditions where hyper-excitability is observed (Chahine, M., Chatelier, A., Babich, O., and Krupp, J. J., Voltage-gated sodium channels in neurological disorders. CNS Neurol Disord Drug Targets 7 (2), p. 144-58 (2008)). Several clinically useful analgesics have been identified as inhibitors of Nav channels. The local anesthetic drugs such as lidocaine block pain by inhibiting Nav channels, and other compounds, such as carbamazepine, lamotrigine, and tricyclic antidepressants that have proven effective at reducing pain have also been suggested to act by sodium channel inhibition (Soderpalm, B., Anticonvulsants: aspects of their mechanisms of action. Eur J Pain 6 Suppl A, p. 3-9 (2002); Wang, G. K., Mitchell, J., and Wang, S. Y., Block of persistent late Na+ currents by antidepressant sertraline and paroxetine. J Membr Biol 222 (2), p. 79-90 (2008)).
The Nav's form a subfamily of the voltage-gated ion channel super-family and comprises 9 isoforms, designated Nav1.1-Nav1.9. The tissue localizations of the nine isoforms vary greatly. Nav1.4 is the primary sodium channel of skeletal muscle, and Nav1.5 is primary sodium channel of cardiac myocytes. Nav's 1.7, 1.8 and 1.9 are primarily localized to the peripheral nervous system, while Nav's 1.1, 1.2, 1.3, and 1.6 are neuronal channels found in both the central and peripheral nervous systems. The functional behaviors of the nine isoforms are similar but distinct in the specifics of their voltage-dependent and kinetic behavior (Catterall, W. A., Goldin, A. L., and Waxman, S. G., International Union of Pharmacology. XLVII. Nomenclature and structure-function relationships of voltage-gated sodium channels. Pharmacol Rev 57 (4), p. 397 (2005)).
Immediately upon their discovery, Nav1.8 channels were identified as likely targets for analgesia (Akopian, A. N., L. Sivilotti, and J. N. Wood, A tetrodotoxin-resistant voltage-gated sodium channel expressed by sensory neurons. Nature, 1996. 379(6562): p. 257-62). Since then, Nav1.8 has been shown to be the most significant carrier of the sodium current that maintains action potential firing in small DRG neurons (Blair, N. T. and B. P. Bean, Roles of tetrodotoxin (TTX)-sensitive Na+ current, TTX-resistant Na+ current, and Ca2+ current in the action potentials of nociceptive sensory neurons. J Neurosci., 2002. 22(23): p. 10277-90). Nav1.8 is essential for spontaneous firing in damaged neurons, like those that drive neuropathic pain (Roza, C., et al., The tetrodotoxin-resistant Na+ channel Nav1.8 is essential for the expression of spontaneous activity in damaged sensory axons of mice. J. Physiol., 2003. 550(Pt 3): p. 921-6; Jarvis, M. F., et al., A-803467, a potent and selective Nav1.8 sodium channel blocker, attenuates neuropathic and inflammatory pain in the rat. Proc Natl Acad Sci. USA, 2007. 104(20): p. 8520-5; Joshi, S. K., et al., Involvement of the TTX-resistant sodium channel Nav1.8 in inflammatory and neuropathic, but not post-operative, pain states. Pain, 2006. 123(1-2): pp. 75-82; Lai, J., et al., Inhibition of neuropathic pain by decreased expression of the tetrodotoxin-resistant sodium channel, Nav1.8. Pain, 2002. 95(1-2): p. 143-52; Dong, X. W., et al., Small interfering RNA-mediated selective knockdown of Na(v)1.8 tetrodotoxin-resistant sodium channel reverses mechanical allodynia in neuropathic rats. Neuroscience, 2007. 146(2): p. 812-21; Huang, H. L., et al., Proteomic profiling of neuromas reveals alterations in protein composition and local protein synthesis in hyper-excitable nerves. Mol Pain, 2008. 4: p. 33; Black, J. A., et al., Multiple sodium channel isoforms and mitogen-activated protein kinases are present in painful human neuromas. Ann Neurol, 2008. 64(6): p. 644-53; Coward, K., et al, Immunolocalization of SNS/PN3 and NaN/SNS2 sodium channels in human pain states. Pain, 2000. 85(1-2): p. 41-50; Yiangou, Y., et al., SNS/PN3 and SNS2/NaN sodium channel-like immunoreactivity in human adult and neonate injured sensory nerves. FEBS Lett, 2000. 467(2-3): p. 249-52; Ruangsri, S., et al., Relationship of axonal voltage-gated sodium channel 1.8 (Nav1.8) mRNA accumulation to sciatic nerve injury-induced painful neuropathy in rats. J Biol Chem. 286(46): p. 39836-47). The small DRG neurons where Nav1.8 is expressed include the nociceptors critical for pain signaling. Nav1.8 is the primary channel that mediates large amplitude action potentials in small neurons of the dorsal root ganglia (Blair, N. T. and B. P. Bean, Roles of tetrodotoxin (TTX)-sensitive Na+ current, TTX-resistant current, and Ca+ current in the action potentials of nociceptive sensory neurons. J Neurosci., 2002. 22(23): p. 10277-90). Nav1.8 is necessary for rapid repetitive action potentials in nociceptors, and for spontaneous activity of damaged neurons. (Choi, J. S. and S. G. Waxman, Physiological interactions between Nav1.7 and Nav1.8 sodium channels: a computer simulation study. J Neurophysiol. 106(6): p. 3173-84; Renganathan, M., T. R. Cummins, and S. G. Waxman, Contribution of Na(v)1.8 sodium channels to action potential electrogenesis in DRG neurons. J Neurophysiol., 2001. 86(2): p. 629-40; Roza, C., et al., The tetrodotoxin-resistant Na+ channel Nav1.8 is essential for the expression of spontaneous activity in damaged sensory axons of mice. J Physiol., 2003. 550(Pt 3): p. 921-6). In depolarized or damaged DRG neurons, Nav1.8 appears to be the primary driver of hyper-excitablility (Rush, A. M., et al., A single sodium channel mutation produces hyper- or hypoexcitability in different types of neurons. Proc Natl Acad Sci USA, 2006. 103(21): p. 8245-50). In some animal pain models, Nav1.8 mRNA expression levels have been shown to increase in the DRG (Sun, W., et al., Reduced conduction failure of the main axon of polymodal nociceptive C-fibres contributes to painful diabetic neuropathy in rats. Brain. 135(Pt 2): p. 359-75; Strickland, I. T., et al., Changes in the expression of Nav1.7, Nav1.8 and Nav1.9 in a distinct population of dorsal root ganglia innervating the rat knee joint in a model of chronic inflammatory joint pain. Eur J Pain, 2008. 12(5): p. 564-72; Qiu, F., et al., Increased expression of tetrodotoxin-resistant sodium channels Nav1.8 and Nav1.9 within dorsal root ganglia in a rat model of bone cancer pain. Neurosci. Lett. 512(2): p. 61-6).
The primary drawback to the known Nav inhibitors is their poor therapeutic window, and this is likely a consequence of their lack of isoform selectivity. Since Nav1.8 is primarily restricted to the neurons that sense pain, selective Nav1.8 blockers are unlikely to induce the adverse events common to non-selective Nav blockers. Accordingly, there remains a need to develop additional Nav channel antagonists preferably those that are more Nav1.8 selective and more potent with increased metabolic stability and with fewer side effects.